A BTP spokeswoman said: “On Saturday, 12 January a 20-year-old man from Lewisham voluntarily attended a London police station in connection with the incident and is helping officers with their enquiries."

“We still need to speak to the woman pictured in the images so we can try to establish exactly what took place. We continue to urge her or anyone who knows her, as well as any passengers who were on the train at the time, to come forward.”